Field of the Invention
The present invention relates to a quantum cascade laser using intersubband transitions in a quantum well structure.
Related Background Art
Light in a mid-infrared wavelength range (for example, wavelengths of 5 to 30 μm) is an important wavelength range in the field of spectrometric analysis. As a high-performance semiconductor light source in this wavelength range, attention has been attracted to quantum cascade lasers (QCL) in recent years (e.g., cf. Patent Documents 1 to 3).
The quantum cascade laser is a monopolar type laser element which uses a level structure including subbands formed in a semiconductor quantum well structure, to generate light by transitions of electrons between the subbands, wherein quantum well emission layers, each of which is formed in the quantum well structure and serves as an active region, are cascade-coupled in multiple stages, thereby enabling realization of high-efficiency and high-output operation. This cascade coupling of the quantum well emission layers is realized by use of electron injection layers for injecting electrons into emission upper levels, so as to alternately stack the quantum well emission layers and the injection layers.    Patent Document 1: International Publication No. WO 2014/018599    Patent Document 2: Japanese Patent Application Laid-Open Publication No. H8-279647    Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2010-521815    Patent Document 4: Japanese Patent Application Laid-Open Publication No. 2011-035139    Patent Document 5: Japanese Patent Application Laid-Open Publication No. 2011-243781    Patent Document 6: Japanese Patent Application Laid-Open Publication No. 2013-098251    Non Patent Document 1: K. Vijayraghavan et al., “Terahertz sources based on Cerenkov difference-frequency generation in quantum cascade lasers”, Appl. Phys. Lett. Vol. 100 (2012) pp. 251104-1-251104-4    Non Patent Document 2: K. Vijayraghavan et al., “Broadly tunable terahertz generation in mid-infrared quantum cascade lasers”, Nat. Commun. Vol. 4 Art. 2021 (2013) pp. 1-7    Non Patent Document 3: R. Kohler et al., “Terahertz semiconductor-heterostructure laser”, NATURE Vol. 417 (2002) pp. 156-159    Non Patent Document 4: S. Fathololoumi et al., “Terahertz quantum cascade lasers operating up to ˜200 K with optimized oscillator strength and improved injection tunneling”, Optics Express Vol. 20 (2012) pp. 3866-3876    Non Patent Document 5: Q. Y. Lu et al., “Room temperature single-mode terahertz sources based on intracavity difference-frequency generation in quantum cascade lasers”, Appl. Phys. Lett. Vol. 99 (2011) 131106-1-131106-3    Non Patent Document 6: Q. Y. Lu et al., “Widely tuned room temperature terahertz quantum cascade laser sources based on difference-frequency generation”, Appl. Phys. Lett. Vol. 101 (2012) pp. 251121-1-251121-4    Non Patent Document 7: Q. Y. Lu et al., “Room temperature terahertz quantum cascade laser sources with 215 μW output power through epilayer-down mounting”, Appl. Phys. Lett. Vol. 103 (2013) pp. 011101-1-011101-4    Non Patent Document 8: Q. Y. Lu et al., “Continuous operation of a monolithic semiconductor terahertz source at room temperature”, Appl. Phys. Lett. Vol. 104 (2014) pp. 221105-1-221105-5    Non Patent Document 9: C. Pflugl et al., “Surface-emitting terahertz quantum cascade laser source based on intracavity difference-frequency generation”, Appl. Phys. Lett. Vol. 93 (2008) pp. 161110-1-161110-3    Non Patent Document 10: Y. Jiang et al., “External cavity terahertz quantum cascade laser sources based on intra-cavity frequency mixing with 1.2-5.9 THz tuning range”, J. Opt Vol. 16 (2014) 094002 pp. 1-9